The present invention relates to aqueous amino resin blends comprising as additives aliphatic amines containing at least one amino group and at least one hydroxyl group, the use thereof as impregnating resins, and a process for the reduction of the formaldehyde emission from impregnates by adding such amines to amino resins.
The faces and edges of woodbase materials are coated using coated foils (finished foils) and impregnated cardboards (edgings), respectively (Kunststoff-Handbuch vol. 10, Duroplaste [Thermosets], Hanser-Verlag, 2nd ed. 1988, p. 464 f., pp. 477 to 479). These finished foils and edgings are produced by impregnating absorbent papers with amino resins (thermosets) such as urea-formaldehyde and/or melamine-formaldehyde resins, for example, in combination where appropriate with aqueous dispersions of thermoplastic resins such as acrylic dispersions or styrene-acrylic dispersions, drying the coated papers in a stream of hot air and at the same time curing the resins, and then coating the impregnated or resinated papers.
Adequate penetration of the impregnation liquors into the paper is achieved if the resin solution is processed from aqueous or alcoholic dilution. Owing to the high level of cellulose swelling in aqueous systems, the foils and edgings produced from high aqueous dilution are brittle, exhibit a high level of water absorption, and even in the coated state have a surface whose aesthetic appeal is low. The procedure described in DE-A 23 09 334, which uses impregnating liquors diluted with C1 to C4 alcohols, does give foils and edgings having good performance properties but requires cumbersome measures for reprocessing the waste air. The route to a solution that is described in DE-A 44 39 156, modifying melamine resins with guanamines, does make it possible to carry out impregnation from purely aqueous liquors. A disadvantage, however, is the significantly higher cost of the amino resin, resulting from the use of the guanamines. In patent application DE-A 198 35 114, there are claimed methyl-etherified aqueous melamine-formaldehyde resins for producing microcapsules to which ammonia or primary, secondary or tertiary amines are added. There is no reporting of the advantageous effect of (hydroxyalkyl)amines on the reduction of formaldehyde emissions from finished foils and edgings.
In EP-A 0 913 412, non-etherified urea-formaldehyde resins are described which have a mass fraction of melamine in the finished resin of from 1 to 5%, the reaction mixture being neutralized, following complete condensation, by the addition of specific tertiary hydroxyalkylamines (preferably triethanolamine, methyldiethanolamine, and dimethylethanolamine). These resins are used as impregnating resins. There is no reporting of the advantageous effect of (hydroxyalkyl)amines on the reduction of formaldehyde emissions from finished foils and edgings.
Formaldehyde emissions of ready-produced finished foils and edgings are determined following storage under standard climatic conditions (23xc2x0 C., 50% relative humidity) in accordance with the standard EN 717-2 (FESYP method, gas analysis). Foils and edgings with formaldehyde emission values of less than 3.6 mg/(hxc2x7m2) by the FESYP method comply with the limit (xe2x80x9cE 1xe2x80x9d) and are referred to below as E1 edgings. The rates of emission found remain more or less constant even after several weeks of storage under standard conditions. Emissions of formaldehyde are due to the use of urea-formaldehyde and/or melamine-formaldehyde resins in the liquors for impregnating the paper or cardboard sheets and/or in the formulations for coating the foils and edgings. By using particularly low-formaldehyde urea-formaldehyde and/or melamine-formaldehyde resins it is possible to reduce the formaldehyde emissions as measured by the FESYP method (standard conditions) to levels of around 2 mg/(hxc2x7m2).
It has surprisingly now been found that when E1 edgings produced in accordance with the prior art with initial formaldehyde emission levels of from 1.0 to 3.5 mg/(hxc2x7m2) are stored, especially under nonstandardized climatic conditions at customary summer temperatures and atmospheric humidities, the formaldehyde emissions rise in the course of a few weeks to levels of in some cases much higher than 3.5 mg/(hxc2x7m2), and so no longer satisfy the E1 criterion. This unexpectedly high increase in formaldehyde emissions was confirmed by storage under defined conditions in a tropical climate (35xc2x0 C., 90% relative humidity), with measurement being carried out only after three-day reconditioning under standard conditions following storage under the tropical climate conditions.
From the prior art it is known that the amount of free formaldehyde and also formaldehyde emissions may be reduced by adding formaldehyde scavengers such as urea and urea derivatives, for example. For example, according to DE-A 38 37 965, finished foils and edgings with formaldehyde emissions that are negligible as determined in accordance with DIN 52368 may be produced by adding urea to the melamine-formaldehyde condensation product. No information is given, however, regarding storage behavior, in particular under tropical conditions.
According to DE-A 34 03 136, mixtures of organic hydroxy compounds and an amide are suitable for use as formaldehyde-binding agents in boards made from wood cellulose materials. The use of these mixtures as formaldehyde scavengers in finished foils and edgings is not described. Addition of the mixtures described in DE-A 34 03 136 to amino resins that are used to produce finished foils and edgings leads to a marked deterioration in the flexibility of the finished foils and edgings produced with them. The use of formaldehyde scavengers known from the literature, such as urea, ethyleneurea, and propyleneurea, resulted in finished foils and edgings which did meet the E1 criterion under standard conditions but which greatly exceeded the E1 limit of 3.5 mg/(hxc2x7m2) under tropical climate conditions.
It is an object of the present invention to provide modified amino resins for producing films and edgings that can be used to produce finished foils and furniture edgings which have formaldehyde emissions reduced significantly in relation to the prior art when stored, particularly also under tropical conditions, but which retain the required performance properties.
This object is achieved by means of aqueous amino resin blends which comprise as additives, aliphatic linear, branched or cyclic amines containing hydroxyl groups.
This invention therefore relates to a process of reducing the formaldehyde emission from finished foils and edgings, wherein aqueous amino resin blends are used for impregnation which comprise aliphatic linear, branched or cyclic amines A having at least one amino group which may be primary, secondary or tertiary, at least one hydroxyl group and from 2 to 20 carbon atoms, and non-etherified or at least partly etherified amino resins B.
Addition of these amines A brings about a reduction of up to 90% in the formaldehyde emissions from the finished foils or edgings (impregnates) as compared with the same resin without added amine.
The present invention also relates to a method of use of amines A having at least one amino group that may be primary, secondary or tertiary, one hydroxyl group and 2 to 20 carbon atoms, comprising adding these amines A to amino resins, and impregnating paper or cardboard with such modified amino resins to reduce formaldehyde emissions from the impregnates.
The invention also relates to an aqueous amino resin blend comprising aliphatic linear, branched or cyclic amines A having at least one amino group which may be primary, secondary or tertiary, at least one hydroxyl group, and 2 to 20 carbon atoms, and non-etherified or at least partly etherified amino resins B, wherein the amino resins have a mass fraction of melamine of less than 1% or more than 5%, in the case where the amines A comprise tertiary hydroxyalkylamines. It is preferred in this case, that the mass fraction of melamine in the amino resins is at least 7%, particularly preferably at least 10%; or at most 0.9%, particularly preferably at most 0.75%, and especially preferably at most 0.5%.
The invention also relates to an aqueous amino resin blend comprising aliphatic linear, branched or cyclic amines A having at least one amino group which may be primary, secondary or tertiary, at least one hydroxyl group, and from 2 to 20 carbon atoms, and at least partly etherified amino resins B.
Such mixtures or blends when used as impregnating agent for the production of finished foils and edgings lead to impregnates (finished foils and edgings) that are distinguished by their markedly reduced formaldehyde emission.
The present invention also relates to a method of use of the aqueous amino resin blends in the production of finished foils or edgings, comprising impregnating paper or cardboard with the said amino resin blends, and subsequently drying the impregnates.
The amines A have preferably from 4 to 12 carbon atoms. The number of hydroxyl groups in the amines A is preferably at least equal to the number of primary, secondary or tertiary amino groups. Suitable amines A are preferably selected from the group consisting of ethanolamine, diethanolamine, bis-2-hydroxypropylamine, bis-3-hydroxypropylamine, triethanolamine, tris-2-hydroxypropylamine, N-methylethanolamine, N-benzylethanolamine, N,N-dimethylethanolamine, 2-hydroxyethylpiperazine, bis-2-hydroxyethylpiperazine, N,Nxe2x80x2-bis(2-hydroxyethyl)diaminoethane, and N,N,Nxe2x80x2,Nxe2x80x2-tetrakis(2-hydroxyethyl)diaminoethane. It is further preferred for the amines A to contain secondary and/or tertiary amino groups, particularly preferably exclusively secondary and/or tertiary amino groups. It is especially preferred that the amines have exclusively secondary amino groups and in particular, the number of hydroxyl groups of these amines is greater by at least 1 than the number of secondary amino groups. Such amines are diethanolamine, bis-2-hydroxypropylamine, bis-3-hydroxypropylamine, N-methylethanolamine, N-benzylethanolamine, 2-hydroxyethylpiperazine, and N,Nxe2x80x2-bis(2-hydroxyethyl)diaminoethane.
The amino resins B are water-soluble melamine-formaldehyde resins, urea-formaldehyde resins or mixed melamine-urea condensates which are non-etherified or at least partly etherified with aliphatic linear or branched alcohols, in particular with C1 to C4 alcohols. The expression xe2x80x9cat least partly etherifiedxe2x80x9d here means that at least 10% of the groups originating from the addition of formaldehyde onto the amino resin former (methylol groups, N-methylene groups and alkoxymethyl groups) are present in the form of alkoxymethyl groups. In the compositions of the invention it is also possible to use those melamine resins in which a fraction (up to 20% of their mass) of the melamine has been replaced by other aminotriazines such as acetoguanamine, caprinoguanamine or benzoguanamine. Preferred resins, however, contain less than 10%, in particular less than 5%, measured on the same scale, of aminotriazines which are different from melamine.
In the amino resins B, the amount of substance of aminotriazines, divided by the sum of the amounts of substance of aminotriazines and urea, is from 0 to 1 mol/mol. The content of bonded formaldehyde per each xe2x80x94NH2 group in the amino resins B, expressed as the ratio of the amount of substance of N-methylol groups and the sum of the amounts of substance of free and of (singly or doubly) methylolated amino groups, is from 0.5 to 2.0 mol/mol, preferably from 0.55 to 1.95 mol/mol, and particularly preferred from 0.6 to 1.9 mol/mol. The content of alkoxy groups originating from the etherifying alcohol, expressed as the ratio of the amount of substance of alkoxy groups and the sum of the amount of substance of free, methylolated, and alkoxymethylated amino groups in the amino resins B, is from 0.0 to 2.0 mol/mol, preferably from 0.1 to 1.95 mol/mol, and particularly preferred from 0.15 to 1.9 mol/mol. Optionally, the resins are at least partly etherified with the aforementioned alcohols, especially methanol, ethanol, n-butanol and isobutanol. Particular preference is given to methanol-etherified amino resins.
The preparation of the amino resins B is widely known. First of all, methylolation and condensation are carried out by adding amino resin formers (melamine, urea, the abovementioned other aminotriazines where appropriate) to formaldehyde at a pH of from 7 to 10 and a temperature of from 40 to 110xc2x0 C., after which the etherifying alcohol is optionally added and reaction is optionally continued at a pH from 1 to 7 and at a temperature from 30 to 80xc2x0 C. The condensation and etherification conditions are guided by the level of water dilutability that is desired for the resin, which is at least 1 part by weight resin to 5 parts by weight deionized water, and by the required penetration properties.
The amino resin blends of the invention may further comprise additives, especially polyethylene glycols C1 and/or aqueous dispersions of a copolymer C2 which preferably contains a mass fraction of at least 50% of units derived from (hydroxy)alkyl (meth)acrylates and/or (meth)acrylic acid. (Hydroxy)Alkyl (meth)acrylates are meant to include, in this context, hydroxyalkyl methacrylates, hydroxyalkyl acrylates, alkyl methacrylates and alkyl acrylates. Furthermore, additions of urea (0.5 to 5 cg/g which is equivalent to 0.5 to 5 g/(100 g)) and polyfunctional alcohols containing from 3 to 6 hydroxyl groups, such as trimethylolpropane, erythritol, pentaerythritol, dimethylolpropane, dipentaerythritol, xylitol, sorbitol and mannitol (0.5 to 7 cg/g which is equivalent to 0.5 to 7 g/(100 g)), have proven advantageous. The stated amount for addition is in each case the mass of the addition based on 100 g of the (undiluted) amino resin. As these additives are added after the condensation of the amino resin, they are not incorporated into the condensation resin.
Suitable polyethylene glycols C1 are oligomers or polymers having (number-average) molar masses Mn of up to about 2000 g/mol. Preference is given to liquid polyethylene glycols having molar masses Mn of from 200 to 600 g/mol. They are used where appropriate in amounts such that the ratio of the mass of C1 to the mass of the solids fraction in the solution or dispersion of the amino resin is between 5 and 50 g/(100 g), preferably from 15 to 45 g/(100 g).
The copolymer dispersions C2 are dispersions of acrylic copolymers in water, preparable for example by emulsion copolymerization of olefinically unsaturated monomers, the monomer mixture used to prepare them preferably containing a predominant fraction (more than 50% of its mass) of acrylic monomers, i.e., acrylic or methacrylic acid and derivatives thereof, especially esters with aliphatic alcohols having from 1 to 10 carbon atoms, and esters with aliphatic polyhydroxy compounds having from 2 to 10 carbon atoms and at least two hydroxyl groups per molecule. Preferred acrylic monomers among the esters are methyl, ethyl, n-butyl, t-butyl, hexyl and 2-ethylhexyl (meth)acrylate, hydroxyethyl (meth)acrylate and hydroxypropyl (meth)acrylate. It is also possible to copolymerize monomers which can be copolymerized, such as styrene and other aromatic vinyl compounds, nitriles of olefinically unsaturated acids such as (meth)acrylonitrile, esters or monoesters of olefinically unsaturated dicarboxylic acids such as maleic acid in particular, vinyl esters such as vinyl acetate or vinyl versatate, vinyl halides or vinyl ethers. These dispersions normally have mass fractions of solids of from 25 to 85%; they are added to the amino resin or else only to the impregnation liquor. The ratio of the mass of copolymer in the copolymer dispersion C2 to the mass of the amino resin B in its aqueous solution or dispersion can be (5 to 150 g): 100 g, preferably (20 to 140 g): 100 g, in the composition.
The finished foils and edgings produced using the amino resin blends of the invention are distinguished by up to 90% reductions in formaldehyde emission under standard and tropical conditions as compared with the prior art. The performance properties of the amino resin blends, and the properties of the finished foils and edgings so produced, are otherwise equivalent to those of the prior art.
In the examples to follow, the invention is further illustrated. There, as well as in the preceding text, all data with the unit xe2x80x9c%xe2x80x9d refer to mass fractions (ratio of the mass of the substance in question, and the mass of the mixture), if not expressly stated otherwise. Values for a concentration in xe2x80x9c%xe2x80x9d are mass fractions of the dissolved substance in the solution (ratio of the mass of solute, divided by the mass of the solution).